Security chiefs on both sides of the Atlantic repeatedly turned down the chance to acquire a vast intelligence database on Osama bin Laden and more than 200 leading members of his al-Qaeda terrorist network in the years leading up to the 11 September attacks, an Observer investigation has revealed.

They were offered thick files, with photographs and detailed biographies of many of his principal cadres, and vital information about al-Qaeda's financial interests in many parts of the globe.

On two separate occasions, they were given an opportunity to extradite or interview key bin Laden operatives who had been arrested in Africa because they appeared to be planning terrorist atrocities.

None of the offers, made regularly from the start of 1995, was taken up. One senior CIA source admitted last night: 'This represents the worst single intelligence failure in this whole terrible business. It is the key to the whole thing right now. It is reasonable to say that had we had this data we may have had a better chance of preventing the attacks.'

He said the blame for the failure lay in the 'irrational hatred' the Clinton administration felt for the source of the proffered intelligence - Sudan, where bin Laden and his leading followers were based from 1992-96. He added that after a slow thaw in relations which began last year, it was only now that the Sudanese information was being properly examined for the first time.

Last weekend, a key meeting took place in London between Walter Kansteiner, the US Assistant Secretary of State for Africa, FBI and CIA representatives, and Yahia Hussien Baviker, the Sudanese intelligence deputy chief. However, although the intelligence channel between Sudan and the United States is now open, and the last UN sanctions against the African state have been removed, The Observer has evidence that a separate offer made by Sudanese agents in Britain to share intelligence with MI6 has been rejected. This follows four years of similar rebuffs.

'If someone from MI6 comes to us and declares himself, the next day he can be in Khartoum,' said a Sudanese government source. 'We have been saying this for years.'

Bin Laden and his cadres came to Sudan in 1992 because at that time it was one of the few Islamic countries where they did not need visas. He used his time there to build a lucrative web of legitimate businesses, and to seed a far-flung financial network - much of which was monitored by the Sudanese.

They also kept his followers under close surveillance. One US source who has seen the files on bin Laden's men in Khartoum said some were 'an inch and a half thick'.

They included photographs, and information on their families, backgrounds and contacts. Most were 'Afghan Arabs', Saudis, Yemenis and Egyptians who had fought with bin Laden against the Soviets in Afghanistan.

'We know them in detail,' said one Sudanese source. 'We know their leaders, how they implement their policies, how they plan for the future. We have tried to feed this information to American and British intelligence so they can learn how this thing can be tackled.'

In 1996, following intense pressure from Saudi Arabia and the US, Sudan agreed to expel bin Laden and up to 300 of his associates. Sudanese intelligence believed this to be a great mistake.

'There we could keep track of him, read his mail,' the source went on. 'Once we kicked him out and he went to ground in Afghanistan, he couldn't be tracked anywhere.'

The Observer has obtained a copy of a personal memo sent from Sudan to Louis Freeh, former director of the FBI, after the murderous 1998 attacks on American embassies in Kenya and Tanzania. It announces the arrest of two named bin Laden operatives held the day after the bombings after they crossed the Sudanese border from Kenya. They had cited the manager of a Khartoum leather factory owned by bin Laden as a reference for their visas, and were held after they tried to rent a flat overlooking in the US embassy in Khartoum, where they were thought to be planning an attack.

US sources have confirmed that the FBI wished to arrange their immediate extradition. However, Clinton's Secretary of State, Madeleine Albright, forbade it. She had classed Sudan as a 'terrorist state,' and three days later US missiles blasted the al-Shifa medicine factory in Khartoum.

The US wrongly claimed it was owned by bin Laden and making chemical weapons. In fact, it supplied 60 per cent of Sudan's medicines, and had contracts to make vaccines with the UN.

Even then, Sudan held the suspects for a further three weeks, hoping the US would both perform their extradition and take up the offer to examine their bin Laden database. Finally, the two men were deported to Pakistan. Their present whereabouts are unknown.

Last year the CIA and FBI, following four years of Sudanese entreaties, sent a joint investigative team to establish whether Sudan was in fact a sponsor of terrorism. Last May, it gave Sudan a clean bill of health. However, even then, it made no effort to examine the voluminous files on bin Laden.